The present invention generally relates to a fuel cell. More specifically, the present invention relates to a fuel cell which utilizes biogenic metabolism.
The fuel cell basically includes a fuel electrode, an oxidizer electrode (or air electrode), and an electrolyte. The principle of its operation is based on the reverse action of the electrolysis of water. That is, the fuel cell receives hydrogen and oxygen and generates water (H2O) and electricity. To be more specific, the fuel electrode is supplied with fuel (hydrogen). Upon oxidation, hydrogen separates into electrons and protons (H+). These protons migrate to the air electrode through electrolyte. At the air electrode, protons react with oxygen supplied thereto, thereby generating water.
The fuel cell converts fuel's energy directly into electrical energy, thereby functioning as a highly efficient electric power generator. It can convert the energy of fossil fuels (such as natural gas, petroleum, and coal) into electric energy very efficiently anytime and anywhere.
Constant efforts have been directed to the research and development of fuel cells for large-scale electric power generation. Indeed, the fuel cell mounted on the space shuttle not only generated electric power but also supplied the crew with water. The fuel cell has proven itself to be a pollution-free electric power generator.
The recent noteworthy development is the fuel cell with a polymeric solid electrolyte which operates at comparatively low temperatures ranging from room temperature to about 90° C. This fuel cell is expected to find use not only as large-scale electric power generator but also as small-scale power source for automobiles and as portable power source for personal computers and mobile equipment.
Unfortunately, the above-mentioned fuel cell with a polymeric solid electrolyte still has problems to be solved despite its advantage of running at comparatively low temperatures. For example, it experiences catalyst poisoning with CO when it runs with methanol as fuel at around room temperature. It needs a catalyst of expensive noble metal such as platinum; it suffers energy loss due to crossover; and it encounters difficulties when it uses hydrogen as fuel.
With the foregoing in mind, there has been proposed an idea of applying biogenic metabolism to fuel cells by noting that biogenic metabolism taking place in an organism is a highly efficient energy conversion mechanism. The term “biogenic metabolism” as used herein embraces respiration, photosynthesis, and the like. Biogenic metabolism has the advantage of excelling in power generating efficiency and proceeding under mild conditions at room temperature.
Respiration is a mechanism consisting of the following steps. First, such nutrients as saccharides, fats, and proteins are incorporated into microorganisms and cells. They pass through the glycolytic and TCA cycles involving several enzymatic reactions. (TCA stands for tricarboxylic acid.) During their passage, they give rise to carbon dioxide (CO2) and reduce nicotinamide adenine dinucleotide (NAD+) into reduced nicotinamide adenine dinucleotide (NADH), thereby converting their chemical energy into oxidation reduction energy or electric energy. The electric energy of NADH is converted directly into electric energy of proton gradient in the electron transfer system. This step is accompanied by the reduction of oxygen that forms water. The thus obtained electric energy forms ATP from ADP with the aid of ATP synthetase. And, this ATP is used for reactions necessary for microorganisms and cells to grow. Such energy conversion takes place in cytosol and mitochondria.
Photosynthesis is a mechanism which consists of steps of taking up light energy and reducing nicotinamide adenine dinucleotide phosphate (NADP+) into reduced nicotinamide adenine dinucleotide phosphoric acid (NADPH) through the electron transfer system, thereby generating electric energy. The result is oxidation of water to give oxygen. This electric energy is used to take up CO2 for carbon fixation and to synthesize carbohydrates.
The biogenic metabolism involves the important NADH generating reaction which is represented by the formula (3) below.Fuel(reduced form)+NAD+Fuel(oxidized form)+NADH+H+  (3)
(substrate) dehydrogenase (product)
So far, there are known hundreds of dehydrogenases. They play an important role as a catalyst that performs highly selective conversion of various substrates into products. Their selectivity stems from the fact that the enzyme consists of protein molecules and hence has a unique three-dimensional structure. It follows therefore that fuel taken into an organism sequentially undergoes reactions involving tens of dehydrogenases until it is oxidized to CO2.
The technical idea of applying the biogenic metabolism to fuel cells has brought forth the microbial cell which takes out electric energy generated by microorganisms through an electron mediator and transfers electrons to the electrodes, thereby producing electric current. See, JP-A No. 2000-133297.
Unfortunately, microbes and cells have not only functions to convert chemical energy into electric energy but also other functions unnecessary for energy conversion. Therefore, the above-mentioned system causes electric energy to be consumed for undesirable reactions, thereby reducing the efficiency of energy conversion.
To cope with this situation, there has been proposed a fuel cell based on an idea of isolating the enzymes and electron mediator involved in reactions from microbes and cells and reconstructing an appropriate environment with them in which desired reactions alone take place. In practice, however, such a fuel cell merely produces a very low current density on account of the slow reaction rate of enzymes.
The present invention provides a fuel cell which uses the biogenic metabolism and yet produces a high current density.